1. Field of the Invention
The invention relates to an optical rotary joint.
2. Description of Relevant Art
Various transmission systems are known for transmitting optical signals between units that are rotatable relative to each other.
An optical rotary joint for a plurality of channels, having a Dove prism as a derotating element is disclosed in US 2005/0036735. Light from supply light-waveguides is coupled via collimators into a Dove prism, derotated by means of the latter, and fed into leading-away light-waveguides via further collimators. In order to render the arrangement independent from the refractive index of a medium in the surroundings of the Dove prism, the prism is provided with supplementary optical members, which ensure perpendicular light entry. Owing to this perpendicular light entry, no refraction occurs at the transition face. A disadvantage of this is the partial reflection of light at the perpendicular transition face, so that at least a portion of the light is coupled back into the light-waveguide. This results in a relatively small return loss.
Another kind of an optical rotary joint is disclosed in WO 01/98801 A2. This describes a micro-optical system having optical and mechanical components as a one-piece constituent part. With this design a substantially higher packing density of the individual glass fibers can be attained. Once again, the parallel light-entry and light exit faces are of disadvantage, as they cause a relatively small return loss.
It is a disadvantage of the known prior art that relatively strong reflections of the transmitted light arise inside the rotary joint or at the collimators. An improvement can be achieved with vapor-coated intermediate layers, the refractive indices of which lie between the refractive index of the optical components and that of the surroundings. However, with different surrounding media such as air or oil it is also necessary to adapt the intermediate layers.
U.S. Pat. Nos. 6,301,405, 5,442,721 and 5,568,578 disclose FORJs that transmit optical beams through a Dove derotating prism element at one-half the rotation rate of a receive optical fiber bundle, in a parallel fashion that permits, in theory, a larger number of optical fiber paths for a given rotary joint length. However, these FORJs are wavelength-dependent and are not particularly well suited for applications in which the FORJs are subject to external pressure, such as in underwater applications.
Summarizing, a common disadvantage of the prior art is that multichannel FORJ using Dove prisms is that different wavelengths have different angles of deflection which leads to a deterioration of the imaging or transmission properties for broadband light signals.